1. Field of the Invention
This invention relates to electronic cardiac pacemakers implantable within the human body and more particularly to cardiac pacemakers which respond to a heart signal to inhibit the pacemaker output and in the absence of a heart signal, provide a regular stimulating signal to the patient's heart.
2. Description of the Prior Art
Heart pacers such as that described in U.S. Pat. No. 3,057,356 issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulus to the heart, whereby it is contracted at a desired rate in the order of 72 beats per minute. Such a heart pacemaker is capable of being implanted in the human body and operative in such an environment for relatively long periods of time. Typically, such pacemakers are implanted within the chest beneath the patient's skin and above the pectoral muscles or in the abdominal region by a surgical procedure wherein an incision is made in the selected region and the pacemaker is implanted within the patient's body. Such a pacemaker provides cardiac stimulation at low power levels by utilizing a small, completely implanted transistorized, battery-operated pacemaker connected via flexible electrode wires directly to the myocardium or heart muscle. The electrical stimulation provided by this pacemaker is provided at a fixed rate.
In an article by D. A. Nathan, S. Center, C. Y. Wu and W. Keller, "An Implantable Synchronous Pacemaker for the Long-Term Correction of Complete Heart Block," American Journal of Cardiology, 11:362, there is described an implantable cardiac pacemaker whose rate is dependent upon the rate of the heart's natural pacemaker and which operates to detect the heart beat signal as derived from the auricular sensor electrode and, after a suitable delay and amplification, delivers a corresponding stimulus to the myocardium and in particular, the ventricle to initiate each heart contraction.
Such cardiac pacemakers, separately or in combination, tend to alleviate some examples of complete heart block. In a heart block, the normal electrical interconnection in the heart between its atrium and its ventricle is interrupted whereby the normal command signals directed by the atrium to the ventricle are interrupted with the ventricle contracting and expanding at its own intrinsic rate in the order of 30-40 beats per minute. Since the ventricle serves to pump the greater portion of blood through the arterial system, such a low rate does not provide sufficient blood supply. In normal heart operation, there is a natural sequence between the atrial contraction and the ventricular contraction, one following the other. In heart block, there is an obstruction to the electrical signal due, perhaps, to a deterioration of the heart muscle or to scar tissue as a result of surgery, whereby a block in the nature of a high electrical impedance is imposed in the electrical flow from the atrium to the ventricle.
Where the heart block is not complete, the heart may periodically operate for a period of time thus competing for control with the stimulation provided by the artificial cardiac pacemaker. Potentially dangerous situations may arise when an electronic pacemaker stimulation falls into the "T" wave portion of each natural complete beat. As shown in FIG. 1, the "T" wave follows by about 0.2 seconds each major beat pulse (or "R" wave causing contraction of the ventricles of the heart). Within the "T" wave is a critical interval known as the "vulnerable period" and, in the case of a highly abnormal heart, a pacemaker impulse falling into this period can conveivably elicit bursts of tachylcardia or fibrillation, which are undesirable and may even lead to a fatal sequence of arythmias.
In U.S. Pat. No. Re. 28,003 of Wilson Greatbatch and assigned to the assignee of this invention, there is disclosed an implantable demand cardiac pacemaker comprising an oscillator circuit for generating a series of periodic pulses to be applied via a stimulator electrode to the ventricle of the heart. The stimulator electrode is also used to sense the "R" wave of the heart, as derived from its ventricle to be applied to a sensing portion of the cardiac pacemaker wherein, if the sensed signal is above a predetermined threshold level, a corresponding output is applied to an oscillator circuit to inhibit the generation of the stimulator pulse and to reset the oscillator to initiate timing a new period.
In FIG. 2, there is shown a portion of the heart pacemaker circuit as disclosed in U.S. Pat. No. Re. 28,003, wherein a suitable voltage source such as a battery, applies positive and negative potentials to buses 14 and 12, respectively. Further, the stimulator electrode is used to couple heart pulses via bus 10 and resistive element 18 to the gate of FET 28. As shown in FIG. 2, the FET 28 is suitably biased by resistive element 24. Further, oppositely disposed diodes 20 and 22 are coupled in-parallel with each other, to the gate of FET 28 to protect the FET from input voltages above and below predetermined levels. The output of FET 28 is applied to the base of transistor 36, suitably biased by resistive elements 30, 37 and 34. The output of the unipolar transistor 36 is in turn coupled by capacitor 38 to the remaining portion of the circuit as described and shown in detail in U.S. Pat. No. Re. 28,003.
As shown in FIG. 1, the human heart beat is a complex wave over the period of each beat and recognizably consists of "P," "Q," "R," "S" and "T" waves. The major and most pronounced pulse is the "R" wave and is normally of a magnitude between 2 and 10mV in the left ventricle, the "T" wave normally follows the "R" wave by approximately 0.2 seconds. The "R" wave typically has a pulse width in the order of 10 to 60msec and due to its relatively small amplitude, requires accurate detection and relatively high amplification to control the oscillator circuit of a demand pacemaker as described above. A significant problem in the use of an FET in such a sensing circuit is its relatively low gain, thus requiring the use of an open-loop type of amplification circuit to achieve the necessary high gain in the order of 300, for example. It is apparent that relatively high gains may be achieved by cascading a plurality of amplifying elements, but in the environment of a cardiac pacemaker, the size and therefore the number of elements that may be incorporated therein is limited. Further, it is difficult without the use of some type of negative feedback to achieve stable operation of the sensing and amplifying circuit and to avoid adjustment of the elements within this circuit to assure the desired frequency and amplitude discrimination. However, due to the limited gain of known FET's, it is difficult to achieve the necessarily high gain and at the same time use negative feedback, which limits the gain of the resulting amplification circuit. Further, it is difficult to bias accurately an FET when used to detect signals at relatively low current levels and further to achieve the desired frequency/amplification characteristics of the entire circuit to detect the heart pulses, without the use of feedback.
Further, it is desired in the design of a sensing portion of cardiac pacemaker circuits to ensure that the circuit as manufactured and adjusted at relatively low temperatures, e.g. a room temperature of 72.degree.F, will detect accurately similar signals at elevated body temperatures, i.e., 98.6.degree.F. In particular, the threshold level detection of the circuit set at the lower temperature may change as the temperature is raised due to differences in temperature coefficients of the biasing elements for the first detection element, an FET. Thus, if the biasing elements have a different temperature coefficient than that of the FET, the relative threshold level of the FET at an elevated temperature within the patient's body will change, thereby detecting signals of other frequencies/amplitudes than desired.